Thin film color filters are typically made by first dispersing dye or pigment in an appropriate medium. Both types of color filters have their respective advantages and disadvantages. Typically, color filters made from dyes do not experience dispersion problems which are commonly observed when the color filters are made from pigments. Therefore, dye-based color filters typically provide superior light transmittance than pigment-based color filters. However, dye-based color filters often exhibit inadequate light, heat and/or chemical stability, thus resulting in relatively short useful lives of these color filters.
U.S. Pat. No. 4,781,444 discloses a dye-based coating composition which incorporates a reactive functional group into a dye molecule, so as to allow the dye molecule to be bonded with the polymer resin. The polymer resin forms the continuous phase of the coating composition. However, the additional step of treating the dye molecules not only increases the production costs, it also causes serious pollution concerns. Furthermore, the addition of a foreign functional group to the dye molecule often causes the dye molecule to change its original light absorption spectrum, thus causing light spectrum of the color filter to be shifted from its intended color.
Pigment-based color filters, on comparison, provide significantly better resistance to light, heat, and/or chemical degradation, relative to the dye-based color filters. The advantage of pigment-based color filters over the conventional dye-based color filters, as far as light, heat, and chemical stabilities are concerned, has been discussed in an article appeared in the 1993 issue of the Journal of SID, vol. 1/3, p 341. In that article, which listed the superior light, heat, and chemical stabilities of pigment-based coating compositions, pigment particles were dispersed in a polymeric resin made from acrylic monomers, such as methyl methacrylate and n-butyl acrylate. However, the pigment particles did not appear to be adequately dispersed in the polymeric resin--to the extent of allowing the desired light transmission is concerned--the light transmittance was measured to be less than 50% using this coating composition.
In order to improve the dispersion of pigment particles in the coating medium so as to improve light transmittance, Japan Laid Open Patent Application 60-129739 teaches a method, by which the coating composition was subject to a 10,000.about.12,000 rpm high speed centrifuge after the pigments were dispersed within the coating composition. Thereafter, the coating composition containing the pigments was filtered through a 1 .mu.m filter device. By these treatments, the light transmittance was improved to above 70%. However, this method has several disadvantages. First, the 10,000.about.12,000 rpm high speed centrifuge is not commonly employed in the industry, and it is not suitable for mass production operations. Second, large amounts of pigments were precipitated after the high speed centrifugation; this not only results in great waste in the pigment raw material, it also makes it very difficult to control the exact amount of pigments to be retained in the centrifuged coating composition, thus causing variations and great quality control problems in the color saturation of the final color filters. Third, the precipitated pigments often cannot be re-used, thus resulting in serious concerns in environmental pollution.